A solid tumor carcinoma in the guinea pig was used as an experimental model to test the hypothesis that increased affinity of a monoclonal antibody, radiolabelled for use as a radiopharmaceutical, would result in increased binding (until saturation) of the antigen sites nearest to the vascular delivery bed and thus prevent uniform penetration of the solid tumor and uniform radiation dosimetry. A mathematical model was used to generate values for the experimental conditions and to predict in vivo results. The actual experiments were done with several different protein amounts of antibody, both control and experimental, and various times of perfusion of the solid tumors. The experimental results validate the model as a useful tool, enhanced understanding of antibody penetration into solid tumors, and gave insights useful in human studies of radiolabeled monoclonal anti-body therapy of human solid cancers.